Pictures: Nine Surprisingly Gassy Cities

Rotterdam, Netherlands

Photograph by Peter Dejong, AP

Workers refurbish a cruise ship in Rotterdam, Netherlands, in 2005. According to a new study, Rotterdam is one of the "dirtiest" cities in the world when it comes to emissions of carbon dioxide (CO2) and methane. The two are by far the most plentiful greenhouse gases emitted by cities and major contributors to global warming, according to study co-author Dan Hoornweg.

The study, which analyzed emissions figures for 100 cities in 33 countries, determined that, for every one of its residents, Rotterdam is responsible for the annual emission of about 29.8 tons of CO2 and methane, combined.

The figure "reflects the large impact of the city's port in attracting industry, as well as fueling of ships," according to the report.

The study looked at how much CO2 and methane a city produces each year based on consumption by residents and industries within city limits. Still, Hoornweg cautions against seeing the report as an indictment of cities in general.

"Differences in production and consumption patterns between cities and citizens mean that it is not helpful to attribute emissions to cities" to the exclusion of suburbs and rural areas, Hoornweg, lead urban specialist at the World Bank in Washington, D.C., said in a statement.

Around the world, emissions per person per year were found to vary from 5 to 25 tons of CO2 and methane for cities in industrialized nations to less than half a ton per person per year in some South Asian cities.

Among the 19 U.S. cities surveyed in the new study, Denver skewed toward the higher end of CO2 and methane emissions. The city was responsible for an annual average of about 21.5 tons of the greenhouse gases for every one resident.

This is more than double the amount for New York City—which was found to have an annual average of about 10.5 tons of CO2 and methane emissions per capita.

One reason for the disparity is density. "Manhattan is dense because it's an island, and that's [also] where people want to be," study co-author Hoornweg told National Geographic News. "So they've developed a very public transportation-friendly system."

In addition to urban sprawl, Denver's relatively high emissions are due to the fact that the city still burns coal to generate most of its electricity, said Chris Kennedy, a civil engineer at the University of Toronto who helped compile the data used in the new study.

Washington, D.C.

Photograph by Tyrone Turner, National Geographic

The U.S. Capitol Building is framed by the smokestacks of the coal plant that still supplies the landmark's electricity. Washington, D.C., had a relatively high per capita greenhouse gas emissions rate: nearly 20 tons per year.

This is mostly due to the fact that the region imports most of its electricity, which is largely generated by coal-burning plants, the University of Toronto's Kennedy said.

The study shows that emissions can vary greatly depending on whether they are calculated according to what a city's residents and institutions consume—as the new study did—versus how much CO2 and methane is produced within city limits.

For example, "from the production perspective, Shanghai has high emissions, but from the consumption perspective, its emissions are much lower," the World Bank's Hoornweg said.

Minneapolis, Minnesota

Photograph by Raymond Gehman, National Geographic

Traders work the phones at the Minneapolis Grain Exchange in Minnesota.

Minneapolis ranked fairly high in greenhouse gas output among the U.S. cities surveyed, with an annual average of about 18 tons of CO2 and methane emissions per resident. That's because, as with Denver, the bulk of Minneapolis's electricity is generated using coal, the University of Toronto's Kennedy said.

In general, city design can have a big impact on its emissions, the study found. For example, residents living in inner city neighborhoods well served by public transportation emit much less carbon dioxide and methane than people living in a city's suburbs.

"Here in the U.S. and Canada, there's a lot of emphasis on spreading out," said study co-author Hoornweg. "But when you sprawl, people get a bigger house, and they get a car and have to commute to work." (See urban sprawl pictures.)

"You just get used to a lifestyle that tends to use more energy and produce more greenhouse gas emissions."

Published February 9, 2011

Calgary, Canada

Photograph by George F. Mobley, National Geographic

Calgary, Canada (pictured after a 1986 blizzard), is responsible for the emission of about 18 tons of CO2 and methane per person per year, according to the study.

Most of the emissions were due to coal-reliant power plants, a commuting culture, and the burning of natural gas for heat in the winter, the University of Toronto's Kennedy said. (See our special report: "The Great Shale Gas Rush.")

Writing on his blog, study co-author Hoornweg said that well-designed neighborhoods can have a bigger impact on emission reductions than banning plastic shopping bags, buying organic food, or even encouraging more fuel-efficient vehicles. (Related: "Plastic-Bag Bans Gaining Momentum Around the World.")

"I don't mean to say that these 'small' activities are meaningless," Hoornweg elaborated in an interview. "I think it's important to do lots of these little things, because the impact of our lives on the environment is the addition of many little things."

Menlo Park, California

Menlo Park emits about 16 tons of CO2 and methane per person per year. Most of that is driven by urban sprawl, due in part to the city's relatively vast area and heavy reliance on automobiles.

"This high percentage is typical for Californian cities," the University of Toronto's Kennedy said. "Los Angeles and Sacramento are similar."

By contrast, California's electricity plants—using mostly hydro-, nuclear, natural gas, or renewable power—generate among the lowest carbon emissions in the United States. Also, California is considered the most energy-efficient U.S. state in terms of electricity use.

Stuttgart, Germany

Photograph by Diane Cook and Len Jenshel, National Geographic

Green roofs adorn buildings in Stuttgart, Germany, which the new study found to have a greenhouse gas emission rate of about 12 tons per person per year. (See more green-roof pictures.) Most of Stuttgart's emissions are due to energy use by industries based in the city.

The study found that many European cities had less than half the emissions per person of cities in the U.S. and Canada.

One reason for this is that many European cities are more compact than their U.S. counterparts. In addition to being good for the environment, high-density development can be beneficial to residents, the authors say.

"Early studies are showing that if you can get where you want faster, you increase your quality of life and your happiness increases," Hoornweg said. For example, "if you can walk where you want to go, you can probably be at your kid's school for more of their games."

Austin, Texas

A woman rests outside an antique shop in Austin, Texas, which had the highest greenhouse gas emission rate of any U.S. city surveyed—about 24 tons per person per year.

Most of those emissions are due to transportation used by Austin's residents and energy used to heat, cool, and power the city's buildings.

The authors say the new study debunks the conventional wisdom that developing nations such as China have cities with the worst carbon footprints. In fact, the opposite is true when carbon footprints are viewed in terms of emissions per person, which gives high density cities such as Beijing an advantage.

It's the world's wealthy cities that are responsible for the highest per capita CO2 and methane emissions, the study says.

"Cities worldwide are blamed for most greenhouse gas emissions, but many cities have very low emissions, as do many city dwellers in even the most industrialized countries," Hoornweg said in a statement.

Published February 9, 2011

Dallas, Texas

Photograph by Joel Sartore, National Geographic

A 52-foot-tall (15.8-meter-tall) blow-up "Big Tex" towers over the State Fair of Texas in Dallas. According to the study, Dallas is responsible for the emission of an annual average of 15 tons of CO2 and methane per resident.

"The amount of cities in the world is about to be doubled in the next 25 years," Hoornweg told National Geographic News. "So it's critical to build those new ones as close to optimum as possible" when it comes to energy use.